Caprock integrity for offshore CO2 storage in the Norwegian North Sea – Seismic investigation of a soft sediment seafloor fracture
The Utsira formation (fm) below the Norwegian North Sea is a promising candidate for CCS due to its big expanse, the relatively easy accessibility and its physical properties, like high porosity and permeability. Situated only about 900m below the seafloor, caprock integrity is a more crucial issue than in deeper lying storage candidates. Earlier studies determined a thick shale layer on top of the Utsira fm, especially in and around the area that presently is used to store CO2. This is also confirmed by the monitoring techniques used to ensure that CO2 stays in the subsurface.
The Hugin Fracture that was discovered in 2011 above the northern part of the Utsira fm is leaking methane-enriched ancient glacial melt water and may question the overall quality of the caprock seal.
In this study, we present recent high-resolution near-offset conventional 3D seismic of an area of about 980km^2 around the fracture as well as high-frequency shallow 2D-seismic lines. We investigate how far the fracture reaches into the subsurface, whether it is connected to other structures that may serve as fluid pathway at least for glacial meltwater and if there is any sign of a fluid migration pathway to the deep subsurface, i.e. the Utsira fm.
Preliminary results show that the Hugin Fracture itself can be traced down to about 50m or 184ms TWT. The fracture is situated above one of many buried tunnel valleys that are criss-crossing the study area in the topmost 500ms TWT of the seismic cube. In addition, there are numerous very shallow bright spots indicating gas pockets, with unknown source of the gas, i.e. biogenic or thermogenic. Some of these bright spots seem to be connected to tunnel valleys or fractures.
Our preliminary results call for a more thorough investigation of the shallow subsurface, especially above the Utsira fm, prior to starting large-scale CCS.
The Hugin Fracture that was discovered in 2011 above the northern part of the Utsira fm is leaking methane-enriched ancient glacial melt water and may question the overall quality of the caprock seal.
In this study, we present recent high-resolution near-offset conventional 3D seismic of an area of about 980km^2 around the fracture as well as high-frequency shallow 2D-seismic lines. We investigate how far the fracture reaches into the subsurface, whether it is connected to other structures that may serve as fluid pathway at least for glacial meltwater and if there is any sign of a fluid migration pathway to the deep subsurface, i.e. the Utsira fm.
Preliminary results show that the Hugin Fracture itself can be traced down to about 50m or 184ms TWT. The fracture is situated above one of many buried tunnel valleys that are criss-crossing the study area in the topmost 500ms TWT of the seismic cube. In addition, there are numerous very shallow bright spots indicating gas pockets, with unknown source of the gas, i.e. biogenic or thermogenic. Some of these bright spots seem to be connected to tunnel valleys or fractures.
Our preliminary results call for a more thorough investigation of the shallow subsurface, especially above the Utsira fm, prior to starting large-scale CCS.
Publisert i AGU, 2013
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